On Some Toinis hi Chemical Geology, 417 



quartzite, or injected among the fissures in adjacent silicious strata. 

 From similar facts, observers in other regions have been led to 

 assign a plutonic origin to certain crystalline limestones. We are 

 thus brought to the conclusion that metamorphic rocks, such as 

 granite, diorite, dolerite, serpentine, and limestone, may under certain 

 conditions, appear as intrusive rocks. The p'asty or semi-fluid state 

 which these rocks must have assumed at the time of their displace- 

 ment is illustrated by the observations of Daubree upon the swelling 

 up of glass and obsidian, and the development of crystals in their 

 mass under the action of heated water, indicating a considerable 

 degree of mobility among the particles. The theory of igneo- 

 aqueous fusion applied to granites by Poulett Scrope and Scheerer, 

 and supported by Elie de Beaumont and by the late microscopic 

 observations of Sorby, should evidently be extended to other intru- 

 sive rocks ; for we regard the latter as being in all cases altered and 

 displaced sediments. 



III. The silico-aluminous rocks of plutonic and volcanic origin 

 are naturally divided into two great groups. The one is represented 

 by the granites, trachytes and obsidians, and is distinguished by 

 containing an excess of silica, a predominance of potash, and only 

 small portions of soda, lime, magnesia and oxide of iron. In the 

 other group silica is less abundant, and silicates of lime, magnesia 

 and iron predominate, together with anorthic feldspars, containing 

 soda and but little potash. To account for the existence of these 

 two types of plutonic rocks, Prof. J. Phillips supposes the fluid mass 

 beneath the earth's crust to have spontaneously separated into a 

 lighter, silicious, and less fusible layer, overlying a stratum of 

 denser basic silicates. In this way he explains the origin of the 

 supposed granitic substratum, of the existence of which however, 

 the study of the oldest rocks affords no evidence. From these two 

 layers, occasionally modified by admixtures, and by partial separa- 

 tion by crystallization and eliquation, Prof. Phillips suggests that 

 we may derive the different igneous rocks. Bunsen and Durocher 

 have adopted, with some modifications, this view ; and the former 

 has even endeavoured to calculate the composition of the normal 

 trachytic and pyroxenic magmas (as he designates the two supposed 

 zones of fluid matter underlying the earth's crust), and then seeks. 

 from the proportion of silica in any intermediate species of rock, to 

 deduce the quantities of alkalies, lime, magnesia and iron which 

 this should contain. 



So long as the trachytic rocks are composed essentially of ortho- 



Canadian Nat. 2 Vol. IV. Xo 6. 



